This question already has an answer here:
- When is a BIG Rewrite the answer? 23 answers
When you begin a new project/function/object you mostly have an idea of the model you want to build. It can be based on the clients' wish, on your ideas for the app or whatever. In the middle you often realise that your model will not work. There may be new requirements, you didn't think of something etc. Then you have two options. Either you rewrite your code to work with the new specifications, or you "hack" the current code to do what you want. A rewrite is time consuming, and you may need to do it several times, but in the long run it often pays. Hacks are fast and often effective for the moment, but many hacks will make the code really bad, and after a while they may come back and bite you in the behind...
How do you determine when to do what?
(Pardon my very non-academic way of explaining this, but I hope you understand what I'm getting at.)